Nearly a decade ago, SeaWorld made headlines around the globe after its star 6-ton, 6.7-meter (22-foot) killer whale Tilikum intentionally killed his trainer after a routine day at the park, an abnormal behavior researchers now say is more than likely due to the fact that the captive animal had spent the last 27 years of his life living in a concrete tank.

By compiling and analyzing a wide array of current scientific data on the welfare of captive orcas into one comprehensive study published in the Journal of Veterinary Behavior, a team of marine mammal scientists, physicians, and a veterinarian conclude that keeping orcas in artificial captive environments significantly increases their risk of illness and early death from chronic stress.

“There has never been a case of a free-ranging orca harming a human in the ocean and yet in captivity, there have been numerous deaths and even more injuries. This hyper-aggression is a function of being in a tank,” lead study author Lori Marino told IFLScience.

Orcas (Orcinus orca) are highly intelligent, wide-ranging apex predators found around the globe. As complex social creatures with structured family systems that rely on each other for hunting and taking care of their young, they exhibit one of the largest and most complex brains in the mammal kingdom. Yet they are the third most common species kept in marine parks and aquariums globally, spending years and even decades living in captivity.

Currently, an estimated 63 are held in captivity despite documented evidence of them exhibiting a range of abnormal behaviors, infections, health conditions, and early death not otherwise seen in natural settings. Simply put: orcas cannot thrive in marine parks.

“Proponents of keeping orcas in marine parks claim that because all their 'needs' are met – they need not travel to find food because it is given to them, eliminating the 'worry' and the 'risks' associated with a free-ranging lifestyle – that they are better off than free-ranging orcas. But this is a deep mischaracterization of who orcas are,” said Marino.

Dawn Brancheau at the Riders of the Storm show in SeaWorld, Orlando, Florida, in 2006. She would be killed by the killer whale Tilikum in 2010. EdSchipul/Wikimedia Commons

Orcas evolved over millions of years to travel far and wide to meet the challenges of finding food and avoiding risks, but when they are not allowed to do this in captivity, they suffer from chronic stress and its effects. To determine how chronic stress hormones impacted their immune system and the brain, researchers examined five factors: physical confinement, sensory disruption, social stress, lack of control, and boredom – all of which are inter-related and cause undue stress on captive orcas.

"The abnormal behaviors observed in captive orcas are simply absent in free-ranging animals. Most captive orcas grate their teeth on hard parts of their enclosure until their teeth are worn down, some to the gums. This kind of behavior doesn’t occur in the wild,” said Marino, adding that free-ranging orcas also do not exhibit behavioral stereotypies (repetitive behaviors like circling) that are seen in captive environments.

Even with routine feeding and around-the-clock veterinary care, Marino says her analysis of captive orcas indicates that there is something “fundamentally incompatible” about keeping an orca in a concrete tank.

“When orcas die in marine parks and aquariums, the response by the facilities is often one of being confounded or stunned. The message is sent that there is no connection between living in concrete tanks and mortality,” explained Marino. “We should not be surprised when a young orca dies in a tank. We know why; it is not a mystery.”

The team notes that their findings call for a “radical shift” in how the cetaceans are treated in order to ensure their complex needs are met.